pH probe interface uP

[vlady_2009]

ph probe interface

Attached image shows schematic of op amp circuit for interfacing pH probe to a ADC input of PIC microprocessor (16F786A). The original circuit (https://www.66pacific.com/ph/ph_1.htm) was designed to input pH probe voltage, use reference gnd of 7V, amplify, invert and display on DMM (acidic pH = negative voltage, neutral pH = 0V, alkaline pH = positive voltage) resulting in the displayed "volts" being equivalent to the pH (a reading of 1 to 14). This works fine, and I have used a reference voltage of 2.5V instead to input to ADC of a PIC.

My "problem" is that when I try to alter the value of R4 (decrease the ohms) to increase possible gain of amplification stage (UB2), don't get desired extra amplification. Have I got it correct that the gain = (R4+RV3)/R4 ?

With RV=10K and R=470ohm max gain about 20 (changing wiper position of pot RV does change the output), giving an "best" output voltage ~1.4V for pH 3 and ~3.6V for pH 10 (giving ~28mV/pH unit - about half expected from theoretical for a pH probe - but my test probe is old and they don't store very well). Since the ADC on the PIC can handle 0-5V, would like to increase the gain the use the extra voltage "headroom" to increase possible resolution of pH reading.

I tried decreasing R4 to 270ohms (max gain ~37, roughly double of existing circuit) but got basically same voltage range for the measured pH range.

Thanks in advance for any advice.

Regards

 

[FvM]

ph probe circuit

With 5V supply voltage, the TL074 output voltage is limited to about 3.5V. Thus your observed "gain" may be possibly the result of a satured output. A rail-to-rail output amplifier would be needed for 5V, single supply operation.

Also an amplifier with lower input current is preferable for a pH instrument.

 

[wp100]

ph probe + amplifiers schematic

Have used this circuit with a standard pic micro - works well.